The present invention relates to a directly heated cathode for a cathode ray tube (CRT), and more particularly, to a cathode structure of an electron gun for a cathode ray tube.
Cathodes for emitting thermions in response to heat energy can be largely divided into indirectly heated and directly heated types, according to the manner in which the emitting source material is heated, with the filament and emitting source being physically separated in indirectly heated cathodes and in contact with each other in directly heated cathodes. The former, which are typically applied to electron guns requiring a great quantity of thermions, includes a sleeve into which a filament is incorporated and a base metal or reservoir fixed to the sleeve.
In the directly heated cathode, on the other hand, the base metal or reservoir is affixed directly to the filament, for application to electron guns for smaller CRTs such as those used in the viewfinder of a video camera. Here, the base metal is coated with oxide material. The reservoir can be applied to a large-screen or industrial CRT requiring a large current, and a typical example is a porous pellet impregnated with cathode material as the thermion-emitting source.
In the conventional directly heated cathode structure, a porous pellet structure directly fixed to the filament has been proposed by the present applicant. As shown in FIG. 1, a pair of filaments 102 are directly in contact with sides of a porous pellet 101 wherein electron-radiating material is impregnated. In such a conventional directly heated cathode structure, according to the type of the filament, the filament 102a is directly welded to the sides of the pellet 101, as shown in FIG. 2, or the filament 102b is fixedly welded such that it passes through the body of porous pellet 101, as shown in FIG. 3.
Also, as another example of the type of the porous pellet directly fixed to the filament, the present applicant has also proposed a cathode structure in which filaments 102 are directly welded to at least three points on the outer sides of a porous pellet 101 impregnated with an electron-radiating material as shown in FIG. 4. In a such a conventional directly heated cathode structure, according to the contact type of the filament, filament 102c is directly welded to at least three points on the sides of the pellet 101, as shown in FIG. 5, or filament 102d is fixed such that it passes through the body of porous pellet 101 crosswise, as shown in FIG. 6.
The above-mentioned directly heated cathode structures need only a very short interval for starting thermionic emission after current is applied and exhibit high-density thermionic emission, since the porous pellet is directly heated by the filament current with the filament in contact with the pellet. However, in the above-described cathode structure, the process of welding the filament to the pellet is difficult to achieve in practice. Accordingly, it is not easy to maintain high quality and the productivity is lowered.